Here's something that I put together and the truth surprised even me, and I'm used to science always being surprising!

-------------------------------------------------

There is a common biological mechanism present even in the simplest of multicellular organisms like Volvox and C.Elegans where the cells that exists in the colony or organism are both male and female at the same time, a hermaphrodite. They can become fully one sex or the other by either male or female side being switched off.

Volvox is a Chlorophyte, or green alga. It exists as a grand spherical colony. Each little alga within the colony bears two flagella, whip-like hairs. The individual alga are connected to each other by thin strands of cytoplasm that enable the whole colony to swim in a coordinated fashion. The individual alga also have small red eye spots.

The colonies even have what we could call a front and rear end. Or, since Volvox resembles a little planet, a 'north and south pole'. In the northern region the eyespots are more developed. This helps the colony to swim towards the light. This differentiation of cells make Volvox quite unique. It is a colony that comes really close to being a multi-celled organism....One of the first things that you notice on Volvox is that most colonies have spheres inside. These are 'daughter' colonies, called gonads. It is a means of asexual reproduction. The gonads grow from cells around the equator of the colony. These cells enlarge and undergo a series of cell divisions until they form a small sphere. There is only one complication, the flagella will be on the inside of the new sphere. It has to turn itself inside out so the flagella will be situated towards the outside of the colony!

Volvox also has a sexual reproduction. As in the asexual reproduction special cells are formed around the equator. These cells develop into special germ cells. Male and female colonies form different germ cells. Sperm cells are formed by division. The female germ cells do not divide. They enlarge to form an ova.

Most species have male and female colonies. In some species the colonies are hermaphroditic. They are able to form sperm as well as ova. But not at the same time so self fertilisation is prevented.

This images shows packages of sperm cells. They will soon swarm out in search of ova in other Volvox.

So there you have it, we just got started and already male and female algae are having sex together. Just need a wet enough environment for the sperm to swim through, either internal for something that lives out of water or external like the volvox. They sense the eggs being there, then go running straight for them.

At the level of complexity around half way there to us are the nematodes like C.Elegans where being an easily switchable to fully one way or the other hermaphrodite is also normal.

Note that in C. elegans hermaphrodite development there is a developmental switch — first sperm are produced and stored in the spermatheca and then oocytes are produced. Oocyte nuclei are produced by meiotic cell division at the distal end of the gonad. They mature in a syncytium--without complete plasma membranes that separate them from one another. The nucleus and cytoplasm are completely enclosed in a plasma membrane later, just prior to fertilization. After fertilization, the eggshell is added. Fertilization takes place as maturing oocytes are squeezed through the spermatheca. Eggs develop in the hermaphrodite body briefly and then are laid through the vulva at about the 40-cell stage.

At the wormbase http://www.wormbase.org/ where they have all kinds of electronic data to crunch through and good links it appears that reproduction with a non-hermaphrodites is more fun than with themselves. Especially in other animals where being only female is also common in which case males/females would need each other more. Less lonely that way too.

Even though it seems like hermaphrodites are somehow an abnormality (as opposed to the way it is with multicellular organisms) there are a good number of them in our human population. Since its a source of gossip and ridicule it would normally be kept their and their doctors secret. But one who was a born music entertainer, appears to be making the best of what they got:

Other than being larger the lower male parts are in females just smaller or look different because it contains just eggs instead of just sperm. And even though totally useless to males, we still have nipples. And the reason is definitely more than decorative. All the mechanisms even to make big breasts to go along with them are all there, just switched off.

We have organs that in a sense have a mind of their own whose purpose in life is to produce more sex cells that when combined produce a new multicellular organism. At the conscious level of awareness it's expressed as "dreaming about sex all day" and certain anatomical functions turning-on to get ready to go at "love at first sight" even though it will only happen in their mind. There is here a feedback mechanism that goes from between the legs to the brain, then back down again. Same thing as controlling a muscle with two neurons to and from but uses hormones on each other too. Thoughts that originate from "down there" in our deepest biology that stays alive by reproducing can be expected to be very hard to stop thinking about.

When in a "relationship" with another mind and body we are fulfilling the purpose of the system which is of course highly behavior related making it possible to be attracted to the same sex or to the first sexual state where both are expressed, especially where you didn't know that they were since it's still possible for them to be "sexy". The success of finding that special person increases the confidence level all the way to the euphoric state of "love" that is so great it produces overconfidence as in it being "blind" that can have negative results.

With the hermaphrodite/male/female sexuality mechanism working the same way for algae to nematodes on up to us and our population having all three represented too co-dependence seems to be the result of the sex chromosomes being great at producing highly reproductive with each other males and females by switching one sex off but not good at all for also producing a highly reproductive hermaphrodite population which is disrupted by the added of a sex chromosome switching mechanism. With it added to the system it's up to the males and females to keep the population going, species is fully male/female co-dependant. Otherwise hermaphrodites that don't need that switching could have big families and the thought of the occasional males and females sneaking off in the woods like they do would be very disgustingly disturbing. But from what I know about behavioral preferences the biological mechanism with the "mind of its own" kicks in and they would have awful thoughts like it might be more fun than all alone with themselves all the time.

Co-dependence of genders would here simply be because of there not being a reproducing hermaphrodite population. At our level of intelligence males and females do such a good job having and rearing babies together (includes benefit of teamwork) that's all the germline sex cells need to keep reproducing on further into time. It's not something that had to "evolve" from something else with no sex it's one of the ways to switch a mechanism that by cell division has two bits of control (one being eggs the other sperm that recombine to produce new organisms) to one of four possible states with 00 being no sex and very sterile and 11 being hermaphrodite and extremely fertile (even to themselves). Male and female would correspond to the states 01 an 10 but I'll let them argue over which binary number should represent each.

It's hard for me to say that co-dependence of genders had to "evolve" when evidence suggests it dates all the way back to the origin of life and oldest known fossils of coral-like colonies while coral are sexual organisms too. It's just one of the possible states that the mechanism is set for, in part because what works for C.Elegans does not work at our level. It takes all the female that a female has just to bear a human child with such a long gestation period and energy needs. Also having to be the macho male expected to be the one to chase the bears out of the cave on top of having to eat to nourish all the male machinery and hormone system along with the babies that regularly pop out would be so physically demanding it would be biologically impossible, in the first place. Can't evolve into something that can't exist.

We are what we are because the design works real good. The self-learning genetic mechanism is very good at finding them by taking good guesses what will work even better. So it still takes a long time to get here (and exact morphology may vary) but living things like us are kept going by a deterministic mechanism that makes us all "meant to be", but like it or not makes sure we know who made who by having molecular ways to push the higher order thought buttons without caring what we think about it.